Acclimated and acute rates of oxygen consumption of the American oyster Crassostrea virginica (Gmelin) were measured under 9 salinity-temperature combinations during declining oxygen tensions. In acclimated and non-acclimated individuals the basal rate of oxygen uptake increased significantly with each 10 C" rise in temperature. Multiple regression equations relating exposure and experimental temperatures and salinities to VO, indicated: (1) as acclimation salinity decreased, the effect of exposure temperature became more pronounced; (2) as acclimation salinity decreased, the effect of exposure salinity decreased; (3) as acclimation temperature increased, the effect of exposure salinity decreased; (4) as acclimation temperature increased, the effect of exposure temperature increased. There is little or no evidence for temperature acclimation even after 3 wk. Q,, values for warm acclimated oysters were generally higher than those of cold adapted oysters with the exception of individuals acclimated at a salinity of 7 %o S and exposed to 28 %O S. At any experimental salinity the highest Q,,, values were observed between 20 and 30°C. C. virginica is well able to regulate VO, when exposed to declining oxygen tensions at all temperature-salinity combinations tested; this capacity decreased considerably at all test temperatures in oysters acclimated tq 7 % S. There is no clear pattern of response between exposure conditions and ability to regulate VO,; the degree of regulation decreases with increasing temperature and/or decreasing salinity. The results indicate that the respiratory physiology of C. virginica is highly adapted to life in a fluctuating environment.

Whether filter-feeding organisms can (1) select, (2) preferentially ingest, and/or (3) preferentially digest suspended particles, is of major importance to our understanding of material flow through marine systems. Investigation has until recently been limited by lack of techniques that can distinguish quantitatively between different particles of the same size. We demonstrate here the ability to distinguish algal food particles quantitatively, even when they are of a similar size, by the detection of their fluorescent photosynthetic pigments using a flow cytometer. The usefulness of this technique is illustrated by experiments on the mussel Mytilus edulis fed a mixed algal diet. In these experiments, a cryptomonad was digested in preference to a dinoflagellate and a diatom.

Measurements were made of resource availabihty and the filtration rate and particle retention efficiency of mussels M y t h s edulis L. from an estuarine site on the River Lynher and from an open coastal site at Whitsand Bay, Cornwall, UK. Small cocci of 0.25 km, large cocci of 0.56 pm and small rods of 0.725 pm mean spherical diameter comprised 87 % of the free-living bacterial population at Whitsand Bay and 86% in the Lynher estuary during September 1984. Free-living bacteria represented only 5 % of the carbon and 9.7 and 11 3 O/O respectively of the nitrogen potentially available for exploitation as a food resource in the water column at the estuanne and open coast sites. Weightspecific clearance rate of particles > 12.7 km diameter by M. edulis from the Lynher estuary was approximately 2.5 1 g-' h-' at 14'C whdst that for mussels from Whltsand Bay was 2.6 1 g-' h-'. Relative retention efficiency of particles was generally similar for mussels from the 2 sites, declining to ca 28 O/O for natural bacterioplankton of 0.513 to 0.524 pm mean spherical diameter. These results suggest no significant intraspecific differences in filtration rates or relatlve particle retention efficiencies, for mussels from the 2 sites, in spite of major differences in partlcle concentrations at the sites. Although ca 65 O/O of the natural bacterioplankton were cleared from the expenmental vessels after 335 min at 14 'C, carbon and nitrogen yield from bacterioplankton at natural concentrations is likely to be small. The complete particle spectrum, represented by living phytoplankton cells, detritus and bacterioplankton, would need to be exploited by the Whitsand Bay population of mussels to sustain carbon and nitrogen demands since the total yleld from all particulate size classes was only 713 ug C h-' and 78 pg N h-' compared with an estimated minimum requirement of 737 pg C h-' and 45.5 ug N h-'. The total carbon and nitrogen resource in the Lynher estuary was well in excess of the estimated requirements of the mussels. However, it is estimated that the free-living bacterial resource would contribute only 4.2 % to the carbon budget and 17 % to the nitrogen budget of the Lynher estuary mussels compared with 1.6 and 7 % respectively of that of the open coast mussels. These results suggest that mussels from both the estuarine and coastal sites utilize phytoplankton or similar sized particles as a primary nutritional resource, although in other systems where bacterial standing stocks are high relative to other particulate resources, bacterioplankton may make a more important contribution to the carbon and nitrogen requirements of filter-feedmg organisms.